The present invention relates to injection molding. More particularly this invention concerns a nozzle assembly used between the outlet end of an injection-molding machine and a mold.
An injection-molding machine typically has a housing forming an elongated cylindrical passage centered on an axis and provided with a screw-like worm that is rotated to move plastic from an intake end to an output end. As the plastic is moved through the passage it is converted from granules or particles by means of heat and pressure to a hot and highly viscous liquid. At the outlet end the plastified material, often polyethylene, is forced into a mold where it is allowed to cool and cure, producing a molded article.
The outlet end of the extruder is normally provided with an adapter or outlet fitting having a passage extending from an inlet end where it has a large diameter to fit with the large-diameter outlet end of the extruder to a small-diameter outlet end that is fitted to the mold. Thus the passage tapers downstream, normally frustoconically.
The problem with such system is that the resin does not flow uniformly through the adapter fitting. It therefore moves slowly along its inner walls and, since the adapter is normally heated to keep the resin liquified, can burn and adhere to these inner walls, forming a deposit. With time the burnt deposit builds up and flakes away, allowing small burnt particles to get into the stream passing through the fitting. When the molded workpiece is being made fully transparent, such inclusions can ruin the looks of the end product, making it unacceptable.
It is therefore an object of the present invention to provide an improved nozzle assembly for an injection-molding extruder.
Another object is the provision of such an improved nozzle assembly for an injection-molding extruder which overcomes the above-given disadvantages, that is which surely eliminates the possibility of the plastified resin burning on the nozzle walls.
A further object is to provide an improved injection-molding system which produces high-quality transparent articles with no burnt-plastic inclusions.
An extruder has an outlet fitting having a passage with a frustoconical portion centered on an axis. According to the invention a flow controller in the passage is formed with a conical extension centered on the axis and fitting complementarily in the frustoconical fitting passage, that is extending downstream relative to the flow direction. An array of axially throughgoing flow passages formed in the controller are spaced angularly about the axis and have frustoconically flared upstream ends. The controller has a radially outwardly projecting flange bearing axially on the outlet fitting and generally blocks the outlet-fitting passage so that all liquefied resin must move through the flow-controller flow passages.
Thus this flow controller prevents flow from slowing down and keeps it all moving rapidly enough that it cannot burn into place before getting into the mold. The construction of the controller is such that there are no pockets where flow is so slow that the plastic can get burnt, eliminating the possibility of burnt inclusions in the finished product. The flow controller can be made simply by standard turning and boring operations.
The controller in accordance with the invention is formed with another conical extension centered on the axis and directed axially oppositely to the extension fitting in the passage, that is upstream.
The downstream extension can according to the invention form with the frustoconical passage portion a frustoconical space of predetermined radial dimension. In this case the flow passages are cylindrical, identical, and of a diameter equal generally to the radial dimension. They are spaced from each other by a distance equal to less than the radial dimension and extend parallel to the axis. Their frustoconically flared upstream ends overlap angularly so that the flow controller presents to the incoming flow virtually nothing but a series of sharp edges, eliminating pockets where the resin can pool and bake onto the body of the flow controller.
The outlet fitting has an annular planar face extending perpendicular to the axis and the flange has a planar end face bearing axially on the planar fitting face. Thus the controller fits tightly on the fitting with no leakage between them.
In another system according to the invention the downstream extension fits snugly in the frustoconical passage portion and the passages are formed as radially outwardly open grooves. The controller is formed on the axis with a central axially throughgoing bore having a frustoconically flared upstream end. The frustoconically flared upstream end of the central bore radially overlaps the frustoconically flared upstream ends of the flow passages.
In accordance with the invention the controller is formed with a seat at the upstream flow-passage ends. The extruder has an end fitting with a downstream end fitting complementarily in the seat. This end fitting has a substantially planar downstream end face bearing axially on the controller and is formed with a radially open passage opening at the downstream end face. More particularly, the seat has a cylindrical inner surface and the downstream end-fitting end has a cylindrical outer surface fitting snugly therewith.